1. Field of the Invention
In most conventional speaker systems an enclosure is needed to contain or control the driver's, i.e., woofer's back wave so it will not destructively interfere with the front wave.
Transmission lines are often folded to conserve space, which creates several persistent problems. For example, the loss of acoustic energy at all frequencies, and also the distortion of the rear wave front. Such disadvantages are caused by abrupt discontinuities encountered by the advancing wave at each bend, i.e., direction change in the line, or reduction in cross-sectional area.
Further, early reflections from rear waves nearest the loudspeaker return to it and are audibly transmitted forwardly through the cone. Also, the pressure on the cone created by early reflections causes the woofer's electrical impedance to vary at different frequencies, thus presenting a more difficult load to the driving electronic amplifier.
2. Prior Art
Transmission lines can cause unwanted resonances which affect sound reproduction. A. R. Bailey is believed to be the first to describe a transmission line in 1965, addressing the problem of unwanted resonances by stuffing the line with acoustically absorbent fibers.
The practice of using a folded line to confine or control the rear wave of a woofer is not new. An early example can be found in U.S. Pat. No. 2,277,525 to Mercurius (1941) which shows a labyrinth of great length and volume having numerous 90.degree. bends. This patent shows a resonant system and makes no comment on the relationship of length to frequency response to keep the rear wave in phase with the loudspeaker for increased sound amplitude at low frequencies.
The U.S. Pat. No. 3,186,589 to Dudodgon (1963) is also a resonant system wherein the baffles are intended to disperse and break up the back wave of the speaker, not to enhance low frequencies in phase with the woofer frequency.
Several embodiments of folded transmission lines are presented in U.S. Pat. No. 3,923,123 to Latimer-Slayer (1975). Both resonant and non-resonant variations are shown, including one in which a single diagonal reflector is introduced as a means to taper the cross-sectional area of the transmission line.
In U.S. Pat. No. 4,128,738 to Gallery (1978), two small 45.degree. corner reflectors are shown in a short line without bends. This patent does not show how any reflection is utilized, nor is the device designed to convert the woofer's hemispherical wave to a planar wave.
U.S. Pat. No. 4,244,269 to Karson (1980) describes variations of a transmission line containing two angled baffles. Although identified as "planar reflectors", these are not positioned so as to guide a planar wave front. According to the patent, col. 3, lines 5-10, the angle of the reflectors is not consistent, and their purpose is mainly to divert the wave into chambers where the sound will be lost, col. 3, lines 20-25.
In the U.S. Pat. No. 4,850,452 (1989) to Wolcott, a bass wave loading uses a labyrinth with radiused curves. Although Wolcott's device is more like a horn than a transmission line, the purpose of the curved surfaces is to provide a smooth internal surface, not to guide planar waves.
The bass loading portion of Harrison's patent, U.S. Pat. No. 4,942,939 (1990), is a folded transmission line utilizing chamfered bends, so called, at corners. In effect it acts on sound waves as Wolcott's did. Harrison states that chamfered bends reduce air turbulence.